Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Light reflection

  1. Nov 23, 2003 #1
    One thing that has allways disturbed my mind is the correct understanding of lightinteractions in nature.

    When we're talking about light reflection (light that is reflected in an arbitrary direction, not like mirrors) is it really reflections or is it new light created by the incoming light that stimulates the material. One other example of this are the flourocerent colors which means that the dissipating wavelength is different that the incoming ones. In this case it must be that the light isn't really reflected but created (and sent out in an arbitrary direction) in the material when light is lit on it. Can anyone please give me a good physically correct answer for this.

    Another thing is the confusion with "black bodies". According to physics the black body should absorb all wavelengths but it should also dissipate all wavelengths. Then how can the body be black? Confusing contradiction.
  2. jcsd
  3. Nov 23, 2003 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Your question is rather vague so I just give you some general information. First find a copy of GED by Richard Feynman, it is a good layman explanation of the fundamental processed which govern what we see.

    There are 2 general types of reflection, specular and diffuse. Specular is what a mirror does, diffuse is what a painted wall does. ALL light is adsorbed and re-emitted by the surface it strikes. Specular reflections are generally generated by conducting surfaces, at the atomic level these surfaces consist of what is called an electron gas. This means that there is a huge number of available energy levels, photons in the visible spectrum interact in a nearly continues fashion, thus creating micro currents which re-emit photons nearly identical to the incident ones.

    In a diffuse reflection the atoms which adsorb the incident photons reemit them in a random direction. The incident photons arrive in a range of energy ranges which excite the atoms to a range of electronic energy levels, upon relaxation there will be a dominate path back to the lower energy levels, this path will determine the color of the surface.

    Please repost your black hole question to the cosmology forum.
    Last edited: Nov 23, 2003
  4. Nov 23, 2003 #3
    Any blackbody at nonzero temperature will radiate with a blackbody spectrum, so it will not look black. It's called a blackbody because the light it radiates is independent of the spectrum of light incident on it: it's not reflected light.

    It wasn't a black hole question.
  5. Nov 23, 2003 #4
    Thanks for the answer. In the case of specular reflection I think you said that the light is actually absorbed and then reemitted by the electron cloud. I can understand in the diffuse case when light is emitted in a random fashion. In specular case, what makes the light reflect then so that the law of reflection really occurs if the light is really absorbed?

    You talk about two cases, diffuse and specular. Are those types of reflections really two completely different cases. You often see materials that are both, meaning that they are somewhat diffuse and somewhat shiny like a mirror. How can a material be both?
  6. Nov 23, 2003 #5


    User Avatar
    Science Advisor

    This is an example of a question which has no sensible answer in quantum mechanical theory. Photons are indistinguishable particles; there is no way to label them, so there is no way to test whether a given light ray emanating from the mirror is the same one which was launched. Look up "indistinguishability" together with "quantum mechanics".
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook